1. Field of the Invention
The present invention relates to a phase voltage measuring device for an Alternating Current (AC) motor and a measuring method thereof, and more particularly to a phase voltage measuring device for an AC motor and a measuring method thereof which can measure a phase voltage being supplied to a motor rotating in the forward and reverse directions according to a control signal supplied to a base terminal of a power switching element for driving the motor.
2. Description of the Prior Art
Generally, a vector control method used for controlling an AC motor makes an AC motor possess a characteristic similar to a Direct Current (DC) motor by converting a stator current of an induction motor to an equivalent value on a rotating d-q axis. The d-axis is positioned to correspond to a flux phase of a rotor that has an equivalent value on the d-axis which is separated into a flux component current, and an equivalent value on the q-axis which is separated into a torque component current.
An AC motor thus driven by the vector control method is widely used in numerically controlled machine tools and robotic systems which call for highly efficient dynamic characteristics and accuracy. In the meantime, in order to realize vector control, an accurate detection of a rotor flux is needed. If only a detection of accurate rotor flux can be achieved, the vector control method has an invariable change of a motor parameter, thereby causing the accurate detection of the rotor flux to have considerable importance. Accordingly, a phase voltage supplied to an AC motor has to be detected in order to detect an accurate rotor flux.
A phase voltage detection device for an AC motor is disclosed in Japanese laid open patent application No. Hei:3-89880 having a control device for an AC motor. As illustrated in FIG. 1, the Japanese laid open patent comprises a plurality of circuits. A servo circuit 1 detects the difference between an objective value Wr* and the present displacement value of Wr of a motor M, and it controls a power control circuit 2 connected electrically to the motor M. A current detection circuit 3 detects currents supplied to respective phases of the motor M. A discrepancy discrimination circuit 4 discriminates discrepancies generated on the motor M according to a comparison between a detection value IR of the current detection circuit 3 and an objective current value IP of the serve circuit 1. A control signal generating circuit 5 supplies a predetermined control signal to the power control circuit 2 according to the discrimination result of the discrepancy discrimination circuit 4. The current detection circuit for detecting respective currents supplied to the motor from the control device of the AC motor as constructed in the foregoing description uses a current measuring coil and a pulse converter, resulting in a difficult installation to the motor and bulkiness.
Accordingly, the present invention takes into consideration the various aforementioned conventional problems. It is an object of the present invention to provide a phase voltage measuring device for an AC motor and a measuring method thereof, which can calculate a rotor flux based on measuring a phase voltage of an invariable motor against a rotor resistance having the most extreme variations among the parameters of the motor.
It is another object of the present invention to provide a phase voltage measuring device and a measuring method thereof, which can reduce the manufacturing cost and can simplify the construction by measuring the phase voltage of the motor in a comparatively simple circuit configuration.